#lang r5rs
(define (multirember_co a lat col)
  (cond
    ((null? lat)
     (col '() '()))
    ((eq? (car lat) a)
     (multirember_co a
                     (cdr lat)
                     (lambda (newlat seen)
                       (col newlat
                            (cons (car lat) seen)))))
    (else
      (multirember_co a
                      (cdr lat)
                      (lambda (newlat seen)
                        (col (cons (car lat) newlat)
                             seen))))))

(define (a-friend x y)
  (null? y))
(define lat '(tuna))
;;(define a (multirember_co 'tuna lat a-friend))